Field of the Invention:
The invention relates to a heat-resistant filter layer made from a material that is at least partially permeable to a fluid, to a filter body having at least one heat-resistant filter layer of this type, and to a process for producing a filter body of this type. The filter bodies are used in particular for the purification of exhaust gases from mobile internal combustion engines used in automotive engineering.
If new vehicle registrations in Germany are considered, it will be found that in 2000 around one third of all newly registered vehicles have diesel engines. By tradition, this percentage is significantly higher than in, for example, France and Austria. The increased interest in diesel vehicles stems, for example, from the relatively low fuel consumption, the currently relatively low prices of diesel fuel, but also from the improved driving properties of vehicles of this type. A diesel vehicle is also very attractive from environmental aspects, since it has a significantly reduced emission of CO2 as compared to gasoline-powered vehicles. However, it should be noted that the level of soot particulates produced during combustion is well above that of gasoline-powered vehicles.
If the purification of exhaust gases, in particular of diesel engines, is considered, it is possible for hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas to be oxidized in a known way by, for example, being brought into contact with a catalytically active surface. However, it is more difficult to reduce nitrogen oxides (NOx) under oxygen-rich conditions. A three-way catalytic converter, as is used, for example, in spark-ignition engines, does not provide the desired effects. For this reason, the selective catalytic reduction (SCR) process has been developed. Furthermore, NOx adsorbers have been tested for use for the reduction of nitrogen oxides.
Discussions have long been ongoing as to whether particulates or long-chain hydrocarbons have an adverse effect on human health, but to date no definitive verdict has been reached. Irrespective of this, it is clearly desirable that emissions of this nature should not be released into the environment above a certain tolerance range. In this respect, the question arises as to what filtering efficiency is actually required in order to be able to comply with the well known statutory guidelines even in the future. If current exhaust emissions from commercially available vehicles in the Federal Republic of Germany are considered, it can be concluded that most passenger automobiles certified under EU III in 1999 are also able to satisfy the requirements of EU IV if they are equipped with a filter with an efficiency of at least 30 to 40%.
To reduce the levels of particulate emissions, it is known to use particulates traps that are constructed from a ceramic substrate. They have passages, so that the exhaust gas that is to be purified can flow into the particulates trap. The adjacent passages are alternately closed off, so that the exhaust gas enters the passage on the inlet side, passes through the ceramic wall and escapes again through the adjacent passage on the outlet side. Filters of this type achieve an efficiency of approximately 95% over the entire range of particulate sizes that occur.
In addition to chemical interactions with additives and special coatings, the reliable regeneration of the filter in the exhaust system of an automobile still constitutes a problem. It is necessary to regenerate the particulate trap, since the increasing accumulation of particulates in the passage wall through which the gas is to flow leads to a constantly increasing pressure loss that has adverse effects on engine performance. The regeneration substantially includes brief heating of the particulate trap and the particulates that have accumulated therein, so that the soot particulates are converted into gaseous constituents. However, the high thermal loading of the particulate trap has adverse effects on the service life.
To avoid this discontinuous regeneration, which is a major factor in promoting thermally induced wear, a system for the continuous regeneration of filters has been developed (CRT: continuous regeneration trap). In a system of this type, the particulates are burnt by oxidation with NO2 at temperatures that are already over 200° C. The NO2 which is required for this purpose is often generated by an oxidation catalytic converter disposed upstream of the particulate trap. However, in particular for use in motor vehicles using diesel fuel, this gives rise to the problem that there is only an insufficient level of nitrogen monoxide (NO) that can be converted into the desired nitrogen dioxide (NO2) in the exhaust gas. Consequently, it has not hitherto been possible to ensure that continuous regeneration of the particulate trap in the exhaust system will occur.
Furthermore, it should be born in mind that, in addition to non-convertible particulates, oil or additional residues of additives also accumulate in the particulate trap and cannot readily be regenerated. For this reason, known filters have to be replaced and/or washed at regular intervals. Filter systems of plate-shaped structure attempt to solve this problem by allowing vibration-like excitation that leads to these constituents being removed from the filter. However, this results in that the non-regeneratable fraction of the particulates in some cases passes directly into the environment without any further treatment.
In addition to a minimum reaction temperature and a specific residence time, it is necessary to provide sufficient nitrogen oxide for the continuous regeneration of particulates using NO2. Tests relating to the dynamic emission of nitrogen monoxide (NO) and particulates have clearly demonstrated that the particulates are emitted in particular when there is no or only a very small amount of nitrogen monoxide in the exhaust gas, and vice versa. What this results in is that a filter with true continuous regeneration substantially has to function as a compensator or store, so that it is ensured that the two reaction partners are present in the filter in the required quantities at a given instant. Furthermore, the filter is to be disposed as close as possible to the internal combustion engine in order to be able to reach temperatures which are as high as possible immediately after a cold start. To provide the required nitrogen dioxide, an oxidation catalytic converter is to be connected upstream of the filter, so as to react carbon monoxide (CO) and hydrocarbons (HC) and in particular also to convert nitrogen monoxide (NO) into nitrogen dioxide (NO2). If the system containing an oxidation catalytic converter and a filter is disposed close to the engine, a suitable position is in particular upstream of a turbocharger which is often used in diesel motor vehicles to increase the boost pressure in the combustion chamber.
If these basic considerations are looked at, the question arises, for actual deployment in automotive engineering, as to how a filter of this type, which in such a position and in the presence of extremely high thermal and dynamic loads has to achieve a satisfactory filtering efficiency, is constructed. In this context, account should be taken in particular of the spatial conditions, which require a new configuration of filters. Whereas the maximum possible volume was to the fore in the case of conventional filters, which were disposed in the underbody of a motor vehicle, in order to ensure a long residence time of the as yet unreacted particulates in the filter and therefore a high efficiency, if the filters are disposed close to the engine, there is not sufficient space or room available.
For this purpose, a new concept has been developed, mainly referred to by the term “open filter system”. The open filter systems are distinguished by the fact that there is no need for the filter passages to be alternately closed off by structural devices. In this context, it is provided that the passage walls are constructed at least in part from porous or highly porous material and that the flow passages of the open filter have diverting or guiding structures. These internal fittings cause the flow and the particulates contained therein to be diverted toward the regions made from porous or highly porous material. Surprisingly, it has emerged that the particulates, as a result of being intercepted and/or impacting, are retained on and/or in the porous passage wall. The pressure differences in the flow profile of the flowing exhaust gas are of importance to this effect occurring. The diversion additionally makes it possible to produce local reduced pressure or excess pressure conditions, leading to a filtration effect through the porous wall, since the abovementioned pressure differences have to be compensated for.
The particulate trap, unlike the known closed screen or filter systems, is open, since there are no flow blind alleys. This property can therefore also be used to characterize particulate filters of this type, so that, for example, the “freedom of flow” parameter is suitable for describing the systems. By way of example, a “freedom of flow” of 20% results in that, when viewed in cross section, it is possible to see through approximately 20% of the surface area. In the case of a particulate filter with a passage density of approximately 600 cpsi (cells per square inch) with a hydraulic diameter of 0.8 mm, this freedom of flow would correspond to a substantially continuous area of over 0.1 mm2. To provide a better explanation, it can also be stated that a particulate filter is referred to as open if in principle particulates can fully pass through it, even particulates that are considerably larger than the particulates that are actually to be filtered out. Consequently, a filter of this type cannot become blocked even in the event of an agglomeration of particulates during operation. One suitable method for measuring the openness of the particulate filter is, for example, to test the maximum diameter of spherical particles that can still trickle through a filter of this type. In the present applications, a filter is open in particular if spheres with a diameter of greater than or equal to 0.1 mm can still trickle through, preferably spheres with a diameter of over 0.2 mm, and in particular spheres with a diameter of more than 0.3 mm.